Histone variant innovation in a rapidly evolving chordate lineage
1 Sars International Centre for Marine Molecular Biology, Norway
2 Department of Biology University of Bergen, Norway
3 Department of Molecular Biology, Nijmegen Center for Molecular Life Sciences, Radboud University Nijmegen, The Netherlands
4 Centre for Cancer Biomedicine, Faculty of Medicine, University of Oslo, and Department of Biochemistry, Institute for Cancer Research, Norwegian Radium Hospital, Oslo University Hospital, Oslo, Norway
5 Department of Molecular Cancer Research, University Medical Center Utrecht, Utrecht, The Netherlands
BMC Evolutionary Biology 2011, 11:208 doi:10.1186/1471-2148-11-208Published: 15 July 2011
Histone variants alter the composition of nucleosomes and play crucial roles in transcription, chromosome segregation, DNA repair, and sperm compaction. Modification of metazoan histone variant lineages occurs on a background of genome architecture that shows global similarities from sponges to vertebrates, but the urochordate, Oikopleura dioica, a member of the sister group to vertebrates, exhibits profound modification of this ancestral architecture.
We show that a histone complement of 47 gene loci encodes 31 histone variants, grouped in distinct sets of developmental expression profiles throughout the life cycle. A particularly diverse array of 15 male-specific histone variants was uncovered, including a testes-specific H4t, the first metazoan H4 sequence variant reported. Universal histone variants H3.3, CenH3, and H2A.Z are present but O. dioica lacks homologs of macroH2A and H2AX. The genome encodes many H2A and H2B variants and the repertoire of H2A.Z isoforms is expanded through alternative splicing, incrementally regulating the number of acetylatable lysine residues in the functionally important N-terminal "charge patch". Mass spectrometry identified 40 acetylation, methylation and ubiquitylation posttranslational modifications (PTMs) and showed that hallmark PTMs of "active" and "repressive" chromatin were present in O. dioica. No obvious reduction in silent heterochromatic marks was observed despite high gene density in this extraordinarily compacted chordate genome.
These results show that histone gene complements and their organization differ considerably even over modest phylogenetic distances. Substantial innovation among all core and linker histone variants has evolved in concert with adaptation of specific life history traits in this rapidly evolving chordate lineage.